Venusian Atmosphere

Chemically, the atmosphere of Venus consists mostly of carbon dioxide (96.5 percent). The remainder is mostly nitrogen. These are organic gases, which might lead one to jump to the conclusion that life—some form of life—may exist on Venus. Indeed, during the 1930s, spectroscopic studies of Venus revealed the temperature of the planet’s upper atmosphere to be about 240 K—close to the earth’s surface temperature of 290 K. Some speculated that the environment of Venus might be a dense jungle.
In the 1950s, radio astronomy was used for the first time to penetrate the dense cloud layer that envelops Venus. It turned out that surface temperatures were not 240 K, but were closer to 600 K. Those temperatures are incompatible with any form of life we know. But the outlook got only worse. Spacecraft probes soon revealed that the dense atmosphere of Venus creates high surface pressure—the crushing equivalent of 90 Earth atmospheres—and that surface temperatures actually top 730 K. And what about those clouds?
On Earth, clouds are composed of water vapor. But Venus shows little sign of water.
Its clouds consist of sulfuric acid droplets.

The Sun Sets on Venus (in the East)

As we’ve seen, Mercury’s peculiar rotational pattern can be explained by its proximity to the sun. But no such gravitational explanation is available for the peculiar behavior of Venus. If at 59 days, Mercury rotates on its axis slowly, Venus is even more sluggish, consuming 243 Earth days to accomplish a single spin. What’s more, it spins backwards! That is, viewed from a perspective above the earth’s North Pole, all of the planets (terrestrial and jovian) spin counterclockwise—except for Venus, which spins clockwise.
Nobody knows why for sure, but we can guess that the rotational peculiarities of Venus were caused by some random event that occurred during the formation of the solar system—a collision or close encounter with another planetesimal, perhaps. A violent collision, like the one that formed the earth’s moon, might have started Venus on its slow backward spin.

Forecast for Venus: “Hot, Overcast, and Dense”

Venus’s thick atmosphere and its proximity to the sun are a cruel combination. The planet absorbs more of the sun’s energy (being closer to the sun than the earth) and because of its heavy cloud cover, is unable to radiate away much of the heat. Even before astronomers saw pictures of the planet’s surface, they knew that it would not be a welcoming place.
Until the advent of radar imaging aboard space probes such as Pioneer Venus (in the late 1970s) and Magellan (in the mid-1990s), the surface of Venus was a shrouded mystery. Optical photons bounce off the upper clouds of the planet, and all we can see with even the best optical telescopes is the planet’s swirling upper atmosphere. Modern radio imaging techniques (which involve bouncing radio signals off the surface) have revealed a Venusian surface of rolling plains punctuated by a pair of raised land masses that resemble the earth’s continents. Venus has no coastlines, all of it’s surface water having long ago evaporated in the ghastly heat. These land masses, called Ishtar Terra and Aphrodite Terra, are plateaus in a harsh waterless world.
The Venusian landscape sports some low mountains and volcanoes. Volcanic activity on the surface has produced calderas (volcanic craters) andcoronae, which are vast, rough, circular areas created by titanic volcanic upwellings of the mantle.
Venus is surely lifeless biologically, but geologically it is very active. Volcanic activity is ongoing, and many astronomers believe that the significant, but fluctuating, level of sulfur dioxide above the Venusian cloud cover is the result of volcanic eruptions. Probes sent to Venus thus far have not detected a magnetosphere; however, astronomers still believe that the planet has an iron-rich core. Scientists reason that the core of Venus might simply rotate too slowly to generate a detectable magnetic field.

“I Can’t Breathe in Mercury!”

Like the earth’s moon, Mercury possesses insufficient mass to hold—by gravitation—an atmosphere for very long. In the same way that mass attracting mass built up planetesimals, so the early planets built up atmospheres by hanging on to them with their gravitational pull. If an atmosphere was ever associated with Mercury, the heating of the sun and the planet’s small mass helped it to escape long ago. Without an atmosphere to speak of, the planet is vulnerable to bombardment by meteoroids, x-rays, and ultraviolet radiation, as well as extremes of heat and cold. In sunlight, the planet heats to 700 K. In darkness, with no atmosphere to retain heat, it cools to 100 K. Despite the absence of atmosphere, regions at the poles of Mercury may remain permanently in shadow, with temperatures as low as 125 K. These regions, and similar regions on the earth’s moon, may have retained some water ice.

Close Encounter with Mercury

If Mercury was difficult for a professional astronomer like Schiaparelli to observe, it is even more challenging for the amateur. It is never farther than 28 degrees from the sun (due to its small orbital radius) and always seen very low in the sky, either in the west just after sunset or in the east, just before sunrise. Because it is visible only close to the horizon, obstacles and atmospheric conditions (light pollution, smog, and turbulence) may often make it impossible to see. Like the moon (and, as we saw in Chapter 2, Venus), Mercury exhibits phases as different fractions of its face are seen to be illuminated by the sun. The best time to see Mercury is at its crescent phase, because it appears largest in the sky at this time. The reason for the variation in size with phase is that when the planet is on the near side of the sun (at a distance of approximately 0.6 A.U. from us), it is backlit and closer and thus appears large. When it is on the far side of the sun, it is fully illuminated (full), is 1.4 A.U. away, and appears smaller. To get a good look at Mercury, you need a telescope, preferably fitted with an eyepiece that offers about 150magnification. It is also possible to see Mercury in the daytime, but this can be dangerous. Because the planet is so close to the sun, there is a real danger that you might accidentally focus on the sun. Doing so for even a moment can permanently damage your eyesight! If you want to look for Mercury during the day, you should consult a good ephemerides guide (see Chapter 17 and Appendix E) and use a telescope fitted with setting circles (see your telescope’s instruction manual and Chapter 17) to locate the planet precisely. For added safety, always keep a solar filter on the telescope until you have precisely located the planet.
Better yet: Restrict your viewing of Mercury to just before sunrise or shortly after sunset.

Lashed to the Sun

In the days before space-based telescopes and probes, earthbound astronomers did the best they could to gauge the rotation of Mercury. The nineteenth-century astronomer Giovanni Schiaparelli observed the movement of what few, indistinct surface features he could discern and concluded that, unlike any other planet’s, Mercury’s rotation was synchronous with its orbit around the sun.
Synchronous orbit means that Mercury always keeps one face toward the sun, and the other away from it, much as the moon always presents the same face to the earth. Technology marches on. In 1965, by means of radar imaging, unavailable to Schiaparelli in the nineteenth century, astronomers discovered that Mercury’s rotation period was not 88 days, but only 59 days. This discovery implied that Mercury’s rotation was not precisely synchronous with its orbit, but that it rotated three times around its axis every two orbits of the sun.

Mercury: The Moon’s Twin

In many ways, Mercury has more in common with the lifeless moon of our own planet than with the other terrestrial planets. Its face is scarred with ancient craters, the result of massive bombardment that occurred early in the solar system’s history. These craters remain untouched because Mercury has no water, erosion, or atmosphere to erase them. The closest planet to the sun—with an average distance of 960,000 miles (1,546,000 km)—
Mercury is difficult to observe from the earth, and can only be viewed near sunrise or sunset.
Its surface, revealed in detail for the first time in images transmitted by such unmanned probes as Mariner 10 (in the 1970s), is pocked with moonlike craters.
Mariner 10 also discovered a weak but detectable magnetic field around Mercury. As a result, astronomers concluded that the planet must have a core rich in molten iron. This contention is consistent with the planet’s position closest to the center of the solar system, where most of the preplanetary matter—the seeds that formed the planets—would have been metallic in composition